This invention generally relates to a method of producing a gypsum plaster board formed of a gypsum core member and porous sheet members such as paper boards bonded on both surfaces of the gypsum core member, and more particularly to a method of preparing starch paste for use in producing gypsum plaster boards.
Traditionally, in production of gypsum plaster boards, raw material of the gypsum core member, that is, calcined gypsum, reinforcing agent such as glass fiber, and weight reducing agent such as surface active agent are mixed with water to obtain gypsum slurry, which in turn is poured between two porous sheet members such as paper boards. Thereafter, the unset gypsum plaster board is molded to a predetermined thickness by a molding roller or the like and is set by hydration at room temperature. The set gypsum plaster board is then cut to a predetermined size by a cutting machine or other suitable means, and it is dried at about 50.degree.-200.degree. C. to evaporate excessive moisture in the gypsum plaster board. The above process is continuous and the productivity of the gypsum plaster boards is dependent on the drying capacity. Thus, the gypsum plaster boards are dried at relatively high temperatures so that usually the productivity may be increased. However, this practice is disadvantageous, because the temperature of the products in a drying machine increases and the adhesion between the gypsum core member and the paper boards bonded to the core member will be deteriorated.
The adhesion between the gypsum core member and the both paper boards are generally obtained by the development and linkage of a crystal of gypsum dihydrate into the fiber of the paper boards in the setting process.
To prevent a crystal of gypsum dihydrate from being calcinated and dehydrated to give a gypsum hemihydrate in drying at high temperatures and to prevent separation of the gypsum core member from both paper boards, starch paste such as denatured starch or dextrin as an auxiliary adhesive, and a hardening modifier are added to the raw material of the gypsum core member. In drying at high temperatures, the starch paste is effective to migrate with water in the gypsum core member toward the interface between the gypsum core member and both paper boards and effective to cover the crystal of gypsum dihydrate due to water retentivity of the starch paste which developed into the fiber of the paper boards so as to prevent the calcination and dehydration of the crystal.
In the prior art, since starch paste is added as it is in A powdered state, the molecular weight of the starch paste rapidly increases and is gelatinated by heat. As a result, the migration of the starch paste toward the interface may be unsufficiently carried out and a large amount of starch remains in the gypsum core member. Accordingly, a large amount of adhesive is disadvantageously required and especially, in the case that a denatured starch or dextrin is used, production of gypsum plaster boards tends to be costly.